This invention relates to optical pyrometers and more particularly to a two-color band ratioing pyrometer having high spatial resolution.
In attempting to measure the temperature of certain critical elements in the hot zone of a silicon crystal-growth furnace with thermocouples, three problems were encountered: measurements were not repeatable to the accuracy required, the thermal distribution was disturbed, and the thermocouples could not be used to measure the temperature of the molten silicon. It therefore became necessary to use an optical pyrometer illustrated in FIG. 1, which avoided those problems and allowed valid temperature measurements, even under conditions that would produce misleading results with a single-color optical pyrometer, such as variation in emissivity for a particular temperature (if the variation is equal in the two color bands), and viewing the target through a furnace window (if the window has equal transmissivity in the two wavelengths).
A two-color pyrometer measures temperature by calculating the ratio of the radiation at two different wavelength (color) bands. By using a ratio, the measurement becomes much less sensitive to absolute radiation falling on the detector than is the case for pyrometers using a single wavelength band. Thus, a two-color pyrometer is desirable in many applications because such instruments are valid over a wide range of sensor illumination.
Referring to FIG. 1, which shows the sensing head for the prior-art pyrometer, an objective lens at the end of a focusing barrel is adjusted so that the focal point of a telescope comprised of the object lens and a field lens with a reticle for sighting the target. The objective lens is an achromatic lens to prevent color distortion.
The radiation from the objective lens passes through the aperture of the housing for the sensing head, and is reflected by a plane mirror that is front coated to reflect the color bands of interest, but allow visible radiation to pass through the mirror to the field lens. An eyepiece is focused on the reticle of the field lens so that it will be visible to the viewer superimposed on the image of the target.
The mirror reflects the incoming radiation in the color bands of interest to a first end of a fiber-optic cable used to guide the radiation to a pair of detector photodiodes, each designed for optimum response to the different one of the two color bands of interest. As noted above, the fiber-optic cable is made up of a bundle of fibers. The result is a cable having a 0.032 inch diameter. The fibers are randomly arranged at the input end relative to the other end and serve to distribute the radiation uniformly over the detector, a factor that contributes to lower detector noise.
A fiber-optic mount holds the face of the input end of the fiber-optic cable perpendicular to the optical axis of the reflected radiation. When the target is focused at the reticle of the field lens, it will also be focused on the input end of the cable. As a consequence, the diameter of the fiber-optic cable determines the sensing field of view and therefore the spatial resolution of the optical pyrometer.
A problem with the prior-art two-color ratioing pyrometer was the inability to spatially resolve small targets and simultaneously maintain the validity of the temperature measurement. If the spatial resolution were restricted to the size of interest (in this case, targets less than 0.030 inch diameter), then the radiation reaching the detectors, and the detector output signal, would be reduced proportionally to the ratio of the squares of the target diameters. When driven by this vastly reduced detector output, detector signal processing electronics lacked the sensitivity to yield valid measurements. This problem was solved by designing and fabricating an entirely new detector signal processing electronics package as described herein.
Another problem was encountered in the optical design of prior-art pyrometers when the spatial resolution was restricted by a simple optical stop at the input end of the optical fiber cable used in prior art to define the spatial resolution. The optical stop restricted the number of optical fibers transmitting radiation to the detector, thus resulting in non-uniform detector illumination and higher detector noise generation. To solve this problem, the prior-art optical design was modified as described herein to assure uniform detector illumination while maintaining the high spatial resolution.